There are almost unlimited applications for the measurement of proportions of gases in a mixture. A particular example is rapid measurement of oxygen and carbon dioxide in the presence of nitrogen for evaluation of metabolic activity in humans or other organisms by indirect calorimetry. Other uses include general laboratory use, monitoring of combustion gases, and monitoring of green house gases.
Commonly, to determine the proportions of N gases in a mixture, N-1 sensors are used, each one specific to a particular gas. This usually implies a multiplicity of instruments and sample paths, in many cases the necessity of making multiple adjustments to equalize delays and response times of the various instruments, the expense of procuring multiple instruments, and the inconvenience of dealing with them. An exception to this is the mass spectrograph, but this instrument is expensive, bulky, and complex. The present invention relates to the measurement of N-1 physical or chemical parameters in a mixture of N gases in order to determine the relative proportions of N gases. The measurements do not need to be specific to a particular gas, but they must be linearly independent in order to provide unique solutions for the proportions of all the gases.
In particular, the invention relates to the measurement of oxygen and carbon dioxide in a mixture of oxygen, carbon dioxide, and nitrogen, by the measurement of the paramagnetic properties of the gas and the speed of sound of the gas. Desirable qualities of gas measurement include long term stability without readjustment, fast response time, low temperature, non-expendable sensor, ruggedness, and low cost relative to current technologies.
Commonly, oxygen is measured by (1) classical paramagnetic method (Pauling method), (2) zirconium fuel cell, (3) magnetic wind method, (4) acoustical methods based on modulated magnetic fields, and (5) wet chemical cell such as the Clark electrode. (1) suffers from slow response and is easily damaged by physical shock. (2) suffers from poor stability, high temperature, need for warm up, and a consumable cell which must be replaced at intervals. (3) suffers from slow response, interference by other gases, and unstable calibration. (4) appears promising, but has not enjoyed significant commercial success, possibly due to poor stability and interference by acoustical noise. (5) suffers from slow speed, depletion, and changing calibration.
The most prevalent method of measuring carbon dioxide is the measurement of infrared absorption. Such instruments work fairly well, but require relatively high temperatures, significant warm up, and frequent calibration.
As already pointed out, the combination of two separate gas sensing technologies to determine the proportional combination of three gases is a deficient method as regards to complication, cost, matching of delay times and response times.